Abstract

Broadband, durable, damage-resistant antireflection coatings have been the subject of a great deal of interest for a variety of applications. The most promising technique for efficient broadband reduction of reflection is the step-down or "inhomogeneous film" method. Design tools are available for normal incidence. Conventional antireflection coatings are prone to damage and are not adequate because the required refractive indices of the layers are not available. Ultrahigh-frequency binary gratings have been shown to exhibit very low reflectivity. The etched grating has a durable substrate, and any refractive index can be synthesized by adjusting the duty cycle. In this work, it is shown that multilevel binary gratings exhibit broadband, low reflectivity in the IR spectrum for any angle of incidence and for both TE and TM polarizations. A design procedure is developed for oblique incidence and for TE and TM polarizations. A comparison between the antireflection properties of two- and three-level binary gratings is presented. Twodimensional binary gratings are shown to offer further reduction in reflectivity for unpolarized light.

© 1990 Optical Society of America